Warning systems, which warn the driver when he leaves the lane, or rather, threatens to leave the lane, are known. For example, European Patent Application No. 1 074 430 discusses a method in which, based on image sensor systems, the lane in which the vehicle is moving is ascertained, and the driver is warned when the vehicle leaves the lane or threatens to do so. However, information on the exact boundary conditions, under which such a warning is given, is not stated.